1. Field of the Invention
The present invention relates to a control apparatus for performing a focus servo for a multi-layered optical disk capable of recording information on a plurality of layers, and more particularly, to a focus control apparatus which moves an objective lens of a pickup to converge or focus a light beam on a specified recording surface.
2. Description of the Related Arts
A conventional focus control apparatus for an optical disk will be first described in connection with a compact disk player (hereinafter called the "CD player") as an example.
FIG. 1 is a block diagram illustrating the configuration of a main portion of a conventional focus control apparatus for a CD player.
For performing a focus servo in the illustrated focus control apparatus, a switching means 110 is first controlled by a servo loop control unit 107 to input an output of a saw-tooth-wave generator circuit 105 to a driving circuit 111. The driving circuit 111 supplies an output in accordance with a saw-tooth-wave input thereto to a focus coil arranged in a movable portion, not shown, of a pickup 112 to move an objective lens integrally formed with the focus coil in the directions closer to and away from a disk 113 (in the direction of the optical axis).
A light source arranged in the pickup 112 forms a light spot on the disk 113 through the objective lens. Reflected light of the light spot is transduced into electric signals by a light receiving element 101 divided into regions A-D. Respective outputs from the divided regions of the light receiving element 101 are summed by adders 102, 103, 104 to generate a focus sum signal SUM. On the other hand, a focus error signal FE is generated by the adders 102, 103 and a subtractor 108.
The focus error signal FE is at zero level when a relative distance between the objective lens included in the pickup 112 and the disk 113 is equal to a reference value (i.e., a reference distance), and has a sigmoid characteristic, i.e., its output level continuously varies in accordance with a displacement from the reference distance.
The generated focus sum signal SUM is compared with a reference value THO by a comparator means 106 which outputs an FOK signal to the servo loop control unit 107 when the focus sum signal SUM is equal to or more than a predetermined level. On the other hand, the focus error signal FE is also compared with the reference value (zero level) by a comparator means 109 which supplies the comparison result to the servo loop control unit 107 as an FZ signal.
The focus servo control unit 107 recognizes a zero-cross point of the focus error signal by the FZ signal input thereto, and controls the switching means 110 so as to input the focus error signal to the driving circuit 111 when the focus sum signal is equal to or more than the predetermined level and the focus error signal is zero-crossing, thus forming a focus servo loop composed of the pickup 112, the light receiving element 101, and the driving circuit 111.
When the focus servo loop is formed, the focus error signal FE is compensated for the phase in an equalizer amplifier 114 and then input to the driving circuit 111 through the switching means 110. The driving circuit 111, based on the focus error signal input thereto, supplies the focus coil of the pickup 112 with a driving signal for driving the objective lens included in the pickup 112 so as to always maintain the relative distance between the pickup and the disk at the reference value.
As described above, in the conventional CD player, the objective lens included in the pickup is driven by the focus coil to control the distance between the pickup and the disk to always remain constant.
On the other hand, increasingly larger recording capacities have been required for recording media in recent years. To meet this requirement, there has been proposed a multi-layer structure disk having a first and second reflective layers each serving as a recording surface (reflective surface), on which information data is recorded, to increase a recording density. In such a two-layered optical disk, the first reflective layer is formed as a translucent film made of a dielectric material.
For reading such a multi-layer structure optical disk, it is necessary to reliably perform a focus servo on either of the first and second reflective surfaces in response to instructions.